So here is what I tried in the last few days:
The ripples have the exact same spacing when printing with 0.1mm as with 0.3mm. The ripples are spread further apart if I speed up the print to 160% print speedI interpret both of these results that the extruder is not at fault but instead it's something with the print head.
To tackle the 26hz vibrations, I tried to remove things from the print head. First the two fans on either side, then the blTouch. Both didn't change anything in this regard. Then I removed the hotend, leaving only the base with the two bearings. This shifted the vibration towards a higher frequency, peaking at 36hz.
barePrintHead.jpg
Then I removed the print head entirely and put the accelerometer to the exact spot where the print head was on the rod that moves in Y-direction. There were no vibration around 26hz
noPrintHead.jpg
Then I designed a completely new print head which was wider (two linear bearing in each axis instead of one) and with the hotend/bltouch higher up in order to get the center of mass closer to the rods. The result (completely assembled) was disheartening as it had the very same vibrations at 25hz as the old print head had.
newPrintHead.jpg
I'm now considering to create and test a setup with direct drive in order to see how a much heavier setup (with the extruder+motor on the print head) affects my situation.
Still I find it curious that it only affects movement in Y direction. The bearings are identical ones (LM8LUU), the rod along the X axis (driving the movement in y direction) is 556mm long, the other one only 450mm but as both are 8mm diameter hardened steel and the print head is quite light I feel like they shouldn't flex enough to cause my problems.
Now to the 80hz oscillation: I read on this site that GT2 belts should be tensioned to 6-8 pounds which is 26.6-35-5 Newton. I measured the tension on my setup acoustically using the android app "BeltTensionMeter". There I learned that my belts were at around 80-90N. Seems I had a slightly wrong feeling about how tense it should be when I set up the system... So I re-tensioned all belts to the recommended 35N. Then, to make the motors run even more smoothly, I changed microstepping for X and Y axes from 64 to 128 microsteps.
Sadly, both changes didn't affect the 80hz oscillations at all.
Currently, I don't consider the 80hz to be a big problem. The visible ripples on my prints are 2mm apart if I print with 50mm/s print speed (as good as I can measure it...). This correlates very much with my 26hz problem as (50mm/s)/(26/s) = 1.9mm. The 80hz would produce 0.625mm wide ripples at 50mm/s and I don't see any of that kind on my prints.
